An intensified charge coupled image sensor comprises an image intensifier tube having a photoemissive cathode on an interior surface of an input window and a charge coupled device (CCD) imager located at the focal plane of the image intensifier tube. Such a structure is described in U.S. Pat. No. 4,355,229, issued to H. S. Zimmerman et al., on Oct. 19, 1982. The Zimmerman et al. patent is incorporated by reference herein for the purpose of disclosure.
The CCD imager is manufactured by producing a number of discrete CCD imagers on a single semiconductor wafer. As disclosed in U.S. Pat. No. 4,266,334, issued to T. W. Edwards et al., on May 12, 1981, if one desires to sense color information, it is necessary to thin the wafer substrate to a thickness of about 10.mu. (microns). However, because of the fragility of the thinned substrate, it is very difficult to test the CCD imagers after they are thinned. The reason is that the test probes tend to break or otherwise damage the thinned substrate. The Edwards et al. patent, referenced above and incorporated by reference herein for disclosure purposes, describes a method which permits numerous thinned CCD imagers to be made on a single larger wafer and then easily tested, cut out and mounted with little risk or breakage. In the Edwards et al. patent, a relatively large silicon wafer with proper doping is processed in the conventional way, using photolithographic techniques, to produce a relatively large number of CCD's on a common thick substrate. The front surface (the surface containing the electrodes) of the wafer is masked, for example, by placing it in a special masking fixture, or by employing a substance, such as a resist. The masking fixture or substance is made to cover the front surface (the one containing the electrodes) of the wafer and the peripheral edge of the back surface of the wafer. Now instead of thinning the individual devices one at a time, as in the prior art, the entire wafer is thinned in a rotary etching bath to the desired thickness over its entire center area, leaving only an unthinned rim around the peripheral edge of the wafer for support. Then a sheet of glass, which fits into the thinned region, is glued to the back surface, as shown in FIG. 1, to provide a laminated structure which is strong and rigid. The individual devices may then be separated from one another by cutting through the glass and thinned substrate between the imagers.
The CCD's thinned by this method cannot be used directly in intensified charge coupled image sensors because the adhesive used to glue the sheet of glass into the thinned region of the wafer adversely affects the formation of the photoemissive cathode within the image intensifier tube, and the glass sheet and adhesive attenuate the electrons emitted from the photoemissive cathode preventing the electrons from entering the CCD. It is therefore necessary to remove the glass and adhesive from the device after testing and before the device can be mounted within the image intensifier tube. Heretofore, in order to remove the glass, the wafer containing a number of CCD's was placed into an etching holder, shown in FIG. 2. A combination of sulfuric acid (H.sub.2 SO.sub.4) and hydrofluoric (HF) acid was put into the top section of the holder to etch the glass sheet. Unfortunately, acid leakage from the top section of the holder to the lower section frequently occurred with the resultant loss of many CCD imagers through contact of the acid with the electrodes of the CCD imagers.